The present invention relates to a method of making a plastic container, and more particularly, a multi-wall plastic container.
Plastic containers are often made by means of a blow molding method. Such a method generally uses a blow mold having two mold halves each with an internal contour which corresponds to the desired final external contour of the container to be made. A single-walled molten parison is inserted between the mold halves of the open blow mold, the blow mold is closed, and the parison is blown to shape by means of pressurized air provided to the interior of the parison to an extent such that it engages the inner walls of the blow mold so as to assume the shape thereof. The pressurized air is fed by means of a blow mandrel or pin which is inserted into the blow mold from one side or end thereof. When the blow mold is closed, one end of the parison is urged against a neck of the blow mandrel so as to provide, after termination of the blow molding process, a respective opening in the container which may serve, e.g. in fuel tanks, to receive the fuel fill pipe. This method allows single-walled containers to be made in a cost-effective manner.
Particularly in fuel tanks, there is a need to make containers having more than one wall or layer of different materials in order to meet various requirements, for example with respect to mechanical stability or fuel vapor non-permeability. Up to now the making of such containers requires complex multi-step manufacturing methods.
A method is provided for forming multi-walled plastic containers in a single blow mold. In general, the method comprises disposing at least two separate parisons simultaneously into a blow mold and forming them to shape one after another by means of a special blow mandrel or pin. A substantially hollow, cylindrical first parison may be disposed around or outside of a substantially balloon or tube shaped second parison in an initially open blow mold. After closing the blow mold, the first or outer parison is expanded outwardly so that its outer surface engages the inner surface of the blow mold to assume the shape thereof, and thereafter, the second or inner parison is blown or outwardly expanded to an extent such that its outer surface at least partially engages the inner surface of the first or outer parison. The outer and inner parisons may be formed of different materials which may desirably impart different properties such as mechanical stability or resistance to permeation of fuel vapor from the container.
Preferably, the outer and inner parisons are inserted into the blow mold from opposite ends of the mold. The inner parison is positioned, before being inserted into the blow mold, upon the blow pin which includes a radially internal air feed and discharge passage communicating with the interior of the inner parison and at least one radially outer air feed and discharge passage communicating with the space between the outer and inner parisons. An end of the outer parison adjacent to the blow pin is urged, when the blow mold is being closed, against an area of the blow pin radially outward of the inner parison and generally radially outward of the at least one radially outer air feed and discharge passage.
When a fuel tank is to be made by the method of the invention, the outer parison, which forms the outer enclosure of the finished container, can be made by an extrusion method from a high density polyethylene (HDPE) in order to provide a tough, durable outer enclosure. The inner parison, which forms the internal layer or enclosure of the container, can be made by an injection molding method from a material which is substantially impermeable with respect to hydrocarbons such as ethylene vinyl alcohol (EVOH) or a PET-type thermoplastic resin. Preferably, the inner parison is heated to a temperature below its melting temperature before it is inserted into the blow mold, for example up to a temperature of 150 to 200xc2x0 C., to facilitate forming the inner parison.
In a preferred embodiment of the invention, when the blow mold has been closed, initially the outer parison is formed to shape while the inner parison is pressurized, by a pressure equal to or less than the pressure used for forming the outer parison to shape, in order to prevent collapse of the inner parison while the outer parison is formed to shape. After the outer parison has been formed to shape, the pressure therein is reduced but preferably some superatmospheric pressure is maintained before the inner parison is formed to shape in order to stabilize the formed outer parison of the container in the mold and to prevent it from collapsing. Thereafter, the inner parison is formed to shape. After cooling for a certain time the pressure in the formed parisons is released, and the finished container can be removed from the blow mold.
In a further preferred modification of the invention, the blow mold has a plurality of preferably rib-like, inwardly extending projections to form corresponding inwardly extending projections of the outer parison when it is formed to shape. The inner parison can be formed to shape to an extent such that its outer surface engages only the rib-like projections on the inner surface of the formed outer parison while a gap between the formed inner and outer parisons remains in the remaining areas.
Objects, features and advantages of this invention include providing a method for forming multi-walled plastic containers which permits the multi-walled container to be formed in a single blow mold, permits various walls of the container to be formed from different materials, permits the walls of the container to be individually formed to shape by a single blow pin, decreases the time and cost required to manufacture a multi-walled plastic container, can provide an air gap between substantial portions of the walls of the container, and is readily adaptable to the formation of plastic fuel tanks having an outer wall designed for strength and durability and an inner wall designed to substantially prevent the permeation to the atmosphere of hazardous hydrocarbon fuel vapors.